In this type of apparatus, there is a volume referred to as the "thermal" volume or the "blast" volume which contains the arcing contacts and which, when the contacts separate, is heated by the arc, thereby causing the pressure in said volume to increase. At the first zero crossing of the current, the gas expands and blasts the arc.
The following difficulties are known to arise when implementing an apparatus of this type:
When interrupting low value currents (e.g. a current that is less than or equal to the nominal current normally carried by the circuit breaker), the rise in pressure may be too small or too large, depending on the size of the blast volume. If the blast volume is large in size, then the pressure rise is small and the blast may be insufficient. If the blast volume is small, then the pressure rise is large, but the duration of the blast may be too short for good efficiency.
In contrast, when interrupting high currents (e.g. short circuit currents), the pressure rise and the heating up of the gas must not be too large since that could prevent interruption taking place.
In order to solve this problem, proposals have been made, in particular in Document EP-A-0 315 505, to provide a blast volume in the form of an interrupting chamber whose volume is variable depending on the value of the current to be interrupted.
This is achieved by replacing the fixed arcing contact conventionally to be found in circuit breakers with a semi-fixed contact linked to a piston which is subject to thrust from an opposing spring.
The stroke of the piston varies according to the value of the current to be interrupted, thereby causing a corresponding variation in the size of the blast volume.
Such apparatus suffers from a drawback.
When interrupting high value currents, the semi-fixed contact rises quickly and completely, with the spring having no limiting effect, thereby causing the arc to be lengthened excessively, which in turn has the effect of heating the blast gas too much, thereby running the dangers of spoiling the interrupting dielectric, of excessively polluting the medium surrounding the arc, and possibly of causing the second interruption to fail during open-0.3s pause-close-open cycles.
A first object of the invention is to enable high currents to be interrupted by puffing using thermal expansion in a thermal expansion volume having the appropriate size for this purpose, while also enabling low currents to be interrupted by puffing, but using only a portion of the thermal full expansion volume.
A second object of the invention is to provide apparatus in which the upwards speed and stroke of the semi-fixed assembly are limited progressively.
A third object of the invention is to provide an apparatus in which cool gas is injected into the arcing contact zone during an engagement operation, thereby considerably improving the operation of the apparatus during operating cycles of the type mentioned above.